1. Field of the Invention
The invention relates generally to the field of holographic interferometry and in particular to an apparatus for evaluating defects in a remote surface while simultaneously inducing stress in the surface.
2. Description of Related Art
A variety of inspection and testing techniques have been devised to evaluate the structural integrity of mechanical parts employed in the aircraft industry. Of special concern is the evaluation of the nature of defects inside fastener holes. Such defects are a major source of degenerative failure resulting from microscopic cracking caused by drilling of the holes and the subsequent fatigue loading. The most widely used technique for evaluating fastener hole defects is eddy current inspection, which employs magnetic fields as an interrogating medium. However, eddy current inspection is restricted by the size of crack that can be detected. Cracks too small to be detected by eddy current inspection can cause problems and present a significant safety hazard. A system is needed which eliminates the threshold size detection restriction suffered by eddy current systems.
An alternative technique for measuring defects manifested by deformation of a structural element under stress is interferometry, and in particular holographic interferometry. Holographic interferometry is based on the property that upon reconstruction of an image capture medium, such as a photographic emulsion or an electronic image storage memory, which has been multiply exposed to form several superimposed holograms, the several corresponding virtual and real images are respectively formed simultaneously and therefore interfere. The interference is indicative of the extent of deformation during the observation period.
A hologram is an image with a well-defined amplitude and phase distribution. The phase of a reflected wavefront contains information on the distance traversed by the wave between the object and the point at which the image is captured. By choosing a suitable wavelength, knowledge of phase differences permits measurement of changes in the position of the point of reflection over distances of less than one wavelength.
The phase of an image is captured by causing the wavefront to interfere with a wavefront of known phase or by causing the wavefront to interfere with itself, for example by using a device known as a shearing interferometer. In a shearing interferometer, interference is caused by splitting a collimated beam and either phase shifting or expanding part of the beam before causing the constituent parts to interfere. Shearing interferometers have the advantage of simplicity and are relatively inexpensive in comparison with systems using external reference beams.
While conventional interferometric systems have been successfully employed for purposes of evaluating external surfaces of large scale metal structures, they are not suitable for use in situations in which the surfaces to be evaluated are located in confined spaces, and which require evaluation on a microscopic level. Such surfaces are found, for example, in drilled fastener holes of the type noted above. While endoscopic probes exist for penetrating into confined spaces, the images produced are generally unsuitable for more than a superficial analysis of the surface in question. A more detailed analysis has heretofore been impossible, due to image processing limitations and the impossibility, using conventional equipment, of applying stress to the surface and capturing a meaningful image of the stressed surface.
While the mechanical strength of the fastener most often offsets the inherent weakness caused by the hole as it becomes a source of degenerative failure from cracking, the role of rivets and bolts would be substantially improved if the health of the hole could be more easily determined, thereby providing more reliable data for fatigue life evaluation. The combination of both stress inducement in the inspected surface, and interferometric evaluation of the deformation caused by the stress has, however, heretofore been impossible to achieve using conventional testing apparatus.